This invention relates to battery chargers in general and in particular to a battery charger providing a timed quick charge for use with rechargeable batteries such as nickel-cadmium batteries. Battery chargers must be capable of recharging batteries in a reasonable period of time without damaging the batteries.
One approach is to charge batteries at a sufficiently low rate so that a battery may be continually charged without damage to the battery cells. A substantial limitation of this approach is the fact that it often requires an extended period of charge time. A typical slow charge or trickle charge approach might use a 0.1 C rate, that is a rate that will require 10 hours of charging or longer to recharge the battery. While a 10 hour charge rate may avoid damage to the cells in the event the charge is continued for an extended time period, this can represent an unacceptably long charge time.
Batteries may be capable of being recharged at a higher rate, as for example, a 0.3 C or three hour rate. The problem with charging at the higher rate is that if the charge is continued for an extended period of time the battery may overheat, which can result in damage or destruction of battery cells.
Various approaches have been utilized to provide for rapid battery charging while preventing battery damage. Prior approaches include monitoring the temperature and/or voltage of the battery for terminating rapid charge. However, these approaches require the addition of monitoring circuitry within the battery and charger. Another known approach switches the charger to a higher charge rate whenever a battery is inserted into the charger. At the end of a predetermined time interval, the charger reverts to a lower or trickle charge rate. While this approach provides a relatively fast charge, potential problems may result where a substantially fully charged battery is placed into the charger, which then automatically goes into a timed rapid or quick charge cycle. It is therefore desirable to provide a charger that does not automatically go into rapid or quick charge and which further does not permit inadvertent extension of the time of the quick or rapid charge cycle.